Recently, in radio communication, particularly, in mobile communication, various information such as images and data other than speech are transmission targets. It is anticipated that demands for higher-speed transmission will increase in the future, and a radio transmitting technique for realizing high transmission efficiency utilizing limited frequency resources more efficiently to perform high-speed transmission is demanded.
OFDM is one of radio transmitting techniques to meet these demands. OFDM is a multicarrier transmitting technique for transmitting data in parallel using a plurality of subcarriers, and is known to have features of realizing high frequency efficiency and reducing inter-symbol interference under multipath environment and be effective for improving transmission efficiency.
Further, OFDM provides the maximum frequency efficiency in multicarrier communication because the frequencies of a plurality of subcarriers in which data is arranged are orthogonal to each other, and enables multicarrier communication with a comparatively simple hardware configuration. For this reason, OFDM is focused upon as a communication method to be employed in cellular scheme mobile communication and various studies upon this communication are underway. Further, with OFDM, to prevent inter-symbol interference (ISI), the rear portion of an OFDM symbol is added to the head of the OFDM symbol as a cyclic prefix (CP). Consequently, the receiving end is able to prevent ISI as long as the delay time of the delay wave stays within the time length of the CP (hereinafter simply “CP length”).
Furthermore, with OFDM, quality varies significantly between subcarriers due to frequency selective fading resulting from multipath. In this case, given that a signal allocated to a subcarrier in a location which is a valley of fading has poor quality and makes demodulation difficult, it is necessary to improve quality such that demodulation is possible.
There is a repetition technique as a technique for improving quality in OFDM. The repetition technique is directed to performing transmission by generating a plurality of the same symbols by repeating (i.e. repetition) a certain symbol and allocating a plurality of the same symbols to a plurality of different subcarriers or different times, so that the receiving end is able to obtain diversity gain by performing maximum ratio combining of these same symbols (see, for example, Non-Patent Document 1).
On the other hand, transmission diversity techniques which are effective to reduce inter-cell interference include a technique of transmitting the same symbol at the same time, by the same frequency, from the radio communication base station apparatuses (hereinafter “base stations”) of a plurality of cells. As a result of such transmission, a radio communication mobile station apparatus (hereinafter “mobile station”) located near the cell edge receives mixed same symbols from a plurality of base stations. Consequently, in cases where OFDM is applied to this transmission diversity technique, in the mobile station located near the cell edge, inter-cell interference is not produced as long as a plurality of the same OFDM symbols transmitted from a plurality of base stations at the same time are received with a time lag within the range of the OP length, and these OFDM symbols are combined and received as OFDM symbols with amplified transmission power, so that it is possible to obtain diversity gain.
Non-Patent Document 1: “Performance Comparisons between OFCDM and OFDM in a Forward Link Broadband Channel,” Noriyuki MAEDA, Hiroyuki ATARASHI, Yoshihisa KISHIYAMA, Mamoru SAWAHASHI, The Institute of Electronics, Information and Communication Engineers, TECHNICAL REPORT OF ISICE, RCS2002-162, August 2002, pp. 95 to 100